Sen. Joe Lieberman told Senate Majority Leader Harry Reid Sunday that he couldn’t support a new Medicare proposal floated as a compromise to the public option, a development that complicates the bill’s path towards passage before the end of the year.

In a meeting in Reid’s office just off the Senate floor, aides said the Connecticut independent reiterated his concerns with the public insurance option and told the Nevada Democrat that he couldn’t support a new plan to allow people as young as 55 to buy into Medicare.

“On Friday, Sen. Lieberman told Sen. Reid that he had problems with the Medicare buy-in proposal,” said Marshall Wittmann, a spokesman for Lieberman. “Sen. Lieberman affirmed that position with Sen. Reid today.”

A Democratic leadership aide confirmed Lieberman’s comments in the face-to-face meeting, which was first reported by the Huffington Post.

Lieberman’s opposition means that Reid may once again be short of the 60 votes needed to break a filibuster if the majority leader moves forward with a health care bill that includes a Medicare buy-in proposal, whose costs are currently being analyzed by the Congressional Budget Office. Both Lieberman and fellow centrist Sen. Ben Nelson (D-Neb.) laid out their concerns Sunday with the Medicare buy-in plan on CBS’ “Face the Nation.”

“On part of it - the so-called Medicare buy-in - the opposition to it has been growing as the week has gone on," Lieberman said. "Though I don't know exactly what's in it, from what I hear I certainly would have a hard time voting for it because it has some of the same infirmities that the public option did.”

Nelson worried that the plan was a “forerunner of single payer,” a label that Sen. Jay Rockefeller (D-W.Va.) strongly rejected.

The opposition from Lieberman and the concerns of centrists adds a new wrinkle to efforts by Reid to strike a compromise – since all 40 Republicans are expected to vote to sustain a filibuster. Reid will need the support of Lieberman, Independent Sen. Bernie Sanders (Vt.) and 58 Democrats to advance the measure.

If he kills a public option or the Medicare buy-in plan, he could lose the support of Sanders and several of the more liberal members of the Democratic conference. But keeping either of those plans, or one that would “trigger” a public option if private insurers don’t hold down costs, would lose Lieberman - forcing Reid to find at least one moderate GOP senator to advance the proposal.

To win the most likely potential GOP defector, Sen. Olympia Snowe (R-Maine), Reid would likely have to make modifications and slow the debate down since she’s signaled that the Senate needs to take more time to deal with the expansive issue.